Somewhere 11,000 years ago, something weird happened to a dog. It got cancer—and the really damn freaky part is that the cancer could survive even outside of its canine host. That unknown dog is long dead now, but its tumor cells have improbably lived on, continuing to sprout on the genitalia of dogs all over the world.

This contagious cancer is called canine transmissible venereal tumor, or CTVT. In this week's issue of the journal Science, geneticists reported on the first sequencing of the tumor, giving some clues as to the origin and spread of CTVT.

Cancer, as you and I usually know it, originates as a mutant cell that replicates out of control in an individual. If someone else's cancer cell somehow gets into my body, my immune system will attack this foreign cell—as it would attack all foreign things. But CTVT has a special trick up its sleeve that lets it evade immune systems. There's only one other cancer in the whole world that is contagious like this: a tumor that is eating the faces of Tasmanian devils.

So CTVT has this special ability to evade immune systems, but it still needs to get from one pooch to another. If you're hidden deep inside some poor dog's liver, good luck getting inside the liver of another dog; but, if you're on genitalia, then boy are you in luck. CTVT grows like pink, cauliflower-esque bumps on the nether regions. The friction of doing it, presumably doggie-style, sloughs off CTVT cells, and the tumor spreads from one dog to another.

Because every CTVT cell today is a descendant of the dog's original tumor, sequencing CTVT actually tells us something about who that first dog was. Scientists at the Wellcome Trust Sanger Institute determined that originator of CTVT was likely an Alaskan Malamute or Husky-like dog living 11,000 years ago.

By comparing the CTVT sequences taken from two different modern day dogs—an Aboriginal camp dog in Australia and an American cocker spaniel in Brazil—the researchers concluded that their most recent common ancestor was 460 years ago. That means CTVT may have only started rapidly spreading around that time, when humans themselves started regularly sailing around the world.

The researchers also found an insane number of mutations in the CTVT tumors—2 million compared to the several thousand in a human tumor. Despite all these mutations, the tumor cells from that original dog have continued to survive and spread. This is, perhaps, the closest we've come to pet immortality. [Science via Nature News]